A female tourist in the Skydeck at the now Willis Tower observation floor in Chicago. Photograph: Alamy

Stepping on to the glass deck that hangs off Chicago's tallest building is already a thrilling – and terrifying – experience for tourists, but one California family had a scare this week when the glass appeared to crack underneath their feet.

Alejandro Garibay was in one of the four deck boxes with his brother and two cousins on Wednesday night when he heard cracking and saw what he thought was the glass splintering.

In fact, it was the protective coating on the glass – as opposed to the glass itself – that cracked, a Willis Tower spokesman confirmed to the Guardian. The coating is designed to crack when something is dropped on it.

"The Ledge was designed with a protective coating that completely covers all glass surfaces to protect against scratches. This coating does not affect the structural integrity of The Ledge in any way. Occasionally, the coating will crack, as it is designed to, in order to protect the surface of the glass," Brian Rehme said in a statement.

Four boxes known as the Ledge extend off the Skydeck at the Willis Tower – formerly the Sears Tower. The enclosures extend out more than four feet from the building's Skydeck and offer visitors a glass-floor view from 1,353ft above the city.

The tower's Skydeck remains open while the protective coating is being replaced. The Ledge, however, has been temporarily closed for a routine inspection and is expected to reopen shortly, according to the statement.

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Vitaliy Raskalov and Vadim Makhorov, known as the Skywalkers, have scaled some of the most recognisable tourist attractions on the planet, taking to the rooftops of Barcelona's Sagrada Família, Cologne's cathedral, the Eiffel Tower and the Notre Dame in Paris